German Patent, DE 32 12 434 C2 discloses monitoring the filling level of a medium by detecting whether the electrically conductive medium generates an electrical contact between a sensor electrode and the walls of a conductive container or a second electrode. Since there is frequent build-up on the measuring probe depending on the medium to be monitored, a guard electrode is used. This electrode surrounds the sensor electrode coaxially and has the same electrical potential as the sensor electrode. Depending on the nature of the build-up, this embodiment poses the problem of generating a suitable guard signal.
German Patent, DE 10 2006 047 780 A1 describes a filling level measuring probe which is insensitive to build-up over a large measuring range. According to the known solution, an amplification unit and a limiting element are provided, with the limiting element being arranged between the output of the amplification unit and the guard electrode. The guard electrode is supplied with a guard signal via the amplification unit and the limiting element, which may, for instance, be an Ohmic resistance. Similarly, the sensor electrode is supplied with the triggering signal. An evaluation unit monitors the filling level starting from the current signal available at the sensor electrode and the triggering signal and/or the guard signal. The amplification unit generating the guard signal is limited by the limiting element. The signal with a limited amplitude is passed on to the sensor electrode as an excitation signal. Next, the sensor electrode picks off a current signal which, together with the triggering signal or the guard signal, is used to monitor the filling level.
In German Patent, DE 10 2008 043 412 A1, a filling level switch with a memory unit is described, with the memory unit saving threshold values for various media stored in a container. When the threshold value for the medium is exceeded or undershot, a switch signal is generated. The threshold level can especially be specified for the measured value with regard to the medium in the container in such a way that build-up does not influence reliable switching. Since build-up distorts the measuring signal and thus renders an incorrect process variable, the threshold value (that determines the switch point) is preferably set in such a way that it is outside the range of the measuring signal that can be reached by build-up. The apparatus in such case may be designed as a capacitive or a conductive filling level meter. Since the apparatus can automatically adjust to various media (e.g. also in the context of cleaning cycles such as CIP and SIP processes) in the container by determining and/or calculating the ideal switch point from the detected characteristics of the media, complex comparison processes usually required upon the change of a medium are not necessary.
The detection of the filling level by a conductive measuring procedure reaches its limits if the medium to be monitored has almost no electrical conductivity (<0.5 μS/cm), or very low electrical conductivity. Any changes in the conductivity of the medium compared to the conductivity of air is then too small to be reliably detected by the measuring electronics. Media that are difficult to monitor using a conductive measuring method include, e.g. distilled water, molasses or alcohols. Other problematic media are those with an electrical conductivity of less than 1 μS/cm and a relative permittivity of less than 20. Oils and fats lie especially in this range.